In many applications that require the transmission of signals from point to point in a plant, it is necessary to isolate various points by means of buffer amplifiers in the signal lines. In many cases, a fair degree of accuracy in the reproduction of the signals, which may be DC analog signals, is required. Accordingly, it is typical for the system designer to choose a commercial operational amplifier for this function that has specifications conforming to the accuracy requirements of the system. However, accuracy requirements may not be the only factors that enter into the designer's choice of amplifier. For example, if there is a possibility that overvoltages will be seen in the signal lines, the designer must be concerned with whether the operational amplifier can withstand the overvoltages expected.
In order to eliminate the overvoltage requirement as a constraint on amplifier choice, designers have employed circuitry external to the operation amplifier to furnish overvoltage protection. One way to accomplish this is to clamp the operational-amplifier output between predetermined threshold voltages by connecting normally back-biased diodes to reference voltage sources. In order to use this type of protection effectively, it is normally necessary to include a large resistance in the output circuit across which the overvoltage potential can be dropped. This does not necessarily increase the output impedance significantly if appropriate feedback is included, but it does severely limit the current capability of the circuit, and the current restriction is a major drawback of this type of protection circuit.